Thermistor as amplifier for low frequency signals



Oct. 10, 1944.

L. w; HussEY 2,360,233

, THERMISTOR AS AMPLIFIER FOR LOWFREQUENCY SIGNALS Filed Dec. 10, 1941 5NORMAL VARIATION #1111001 COMPENSATION n ranswrrnlsronnou 1 a" cousm-r04m FIG- 9B a (musmr use. as.

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ATTORNEY and particularly amplifying prescribed manner.

'EWRHHSTOR AS R FQR LGW FREQUENEY SEGNALS Luther W. Hussey, Madison, N..l'., assignor to Bell Telephone Laboratories; Incorporated, New York,N. Y; a corporation of New York Application December 14 1941, Serial No.322316 7 Claims.

This invention relates to amplifying systems systems operative over aband of frequencies. More particularly it relates to the use ofsemiconductors such as silver sulphide as amplifying elements in suchsystems and as elements to control the behavior of these amplifyingelements as the ambient temper'ature' varies.

It is known that certain compounds which are ordinarily'classed assemiconductors exhibit two very useful phenomena-negative temperaturecoefficient of resistance and a voltage-current characteristic thatrises to a voltage maximum after which the voltage falls off as thecurrent increases thus achieving a negative slope which is practicallylinear-over a part of its range. It

is in this latter range that these elements f-unction as negativeresistances to facilitate amplification in the manner well-known in theart. Also it ,is because of the negative temperature coeflicienttheydisplay that they are often .usedas temperature controls especiallyto regulate electrical devices in accordance with variations in ambienttemperature. Both of these desirable properties are made use of in theoperation of this invention.

In using semiconductors for' amplification a direct current potential isapplied to bias the element into operation over the most linear portionof its negative resistance region so that when it is inserted in asuitable alternating current circuit maximum undistorted amplificationis realized. Also, in order to increase the frequency range of theamplification band'and the amount of amplification realizable both ofwhich factors are limited by thethermal inertia of thesemiconductor, itis desirable to keep the ele- .ing various waysof using one or moreadditional thermistors to modify the action of the amplifier thermistorin accordance withvthis invention; and

Figs. 9a. and 9!) show typeslof impedance used to compensate thevariation with frequency of amplifier 'thermistors.

Referring,.now, more particularly to Fig. 1 curves l, 2, 3, l, and 5represent a family of thermistor-voltage-current characteristics, curve3 representing the mean, curves l and {representing the effect of highambient temperature and curves 4 and 5 representing the-effect of lowambient temperature. The curve marked 1/R represents the normalvariation of the bias without compensation. Itis the E--I characteristicof a fixed resistance inserted in'series with the thermistor tostabilize its bias. In isthe direct,

current bias, Eb'i's the battery voltage, and V0 a slope equal to thereciprocal of the bias stabilizing resistance is a familiar one and isvalid for any kindof resistive device in series with a linear resistanceand a battery. In the case of the thermistor it is necessary that thebias ments small in volume in order that they may be sensitivelyresponsive to rapid changes in current and temperature.

In using semiconductors as temperature controls each is chosen to havethe proper dimensions and temperature coemcient to regulate theparticular variable feature of the amplifier semiconductor that it isdesigned to control in a Most of these have been discussed in the priorart. Throughout this de- 'scription the word semiconductorand itscommercial synonym, thermistor, will be used interchangeably. 4

For a detailed description of this invention and of the componentembodiments thereof, reference is made to the drawing, in which:

Fig. 1 shows graphically the variable behavior of a thermistor unitoperated in the negative stabilizing resistance and the battery voltagebe sumciently large so that the straight line with slope equal to thereciprocal of the bias stabilizing resistance intersects the thermistordirect current voltage characteristic at one point only. Otherwisethebias will slide to a positive resist ance intersection. Curve a'obrepresents the. locus to be followed by the shifting bias if the gain isto remain constant. a."-ob represents the locus of the bias shift if thedistortion is to remain constant.

. Fig. 2 shows T1 as a negative resistance thermistor for amplifying thesignals generated by source I after which they. are impressed upon thewindings of transformer B from which they are-passed along to load 9.The energy for heating and biasing T1 to its negative resistancecondition is derived from direct current source 1. Its path is fromdirect current source I through the primary winding of transformer 8through starts from source. I, goes through source imcharacteristic inthis manner more of retard coil 4. Condenser 3 is merely a blockingcondenser to prevent direct current energy from entering the signalsource circuit. Its impedance is made sufliciently low so as not toaffect transmission.

Fig. 3 shows a single frequency amplifier without a transformer. Likenumbered elements perform similar functions to those of Fig. 2. Thetuned circuit composed of condenser l and inductance ll could obviouslybe replaced by a band-pass network. In this circuit the thermistoramplifier direct current supply is prevented from entering any part ofthe signal circuit by condenser 3.

With circuits such as those shown in Figs. 2 and 3 the thermistor T1would .be "initially biased to operate about point 0 on voltage-currentcharacteristic 3 of Fig. 1. This operating point is determined by thenegative resistance desired and the required freedom from distortion,This is accomplished by adjusting the direct current source I, which hasa battery voltage Eb, to produce a drop Vo across T1 causing current Into flow through thermistor T1 which will then have its optimum bias formaximum undistorted amplification. When signals from the source I areapplied to T1, in its biased condition, they will cause the voltageacross and current through T1 to oscillate along the tangentialdirection at point 0 on curve 3 provided the frequency of does notexceeda limiting maximum. By operating over the negative resistance portion ofthe alternating current power is transferred to the load than isreceived fromthe signal source, the excess being absorbed from thesource of direct current supply in a manner somewhat similar to that ofa vacuum tube amplifier. T1 thus provides the negative resistancenecessary for very satisfactory ampliflcation over a frequency/rangelimited only by its thermal lag which can be reduced by reducing themass of T1. But if the ambient temperature rises, the directv currentvoltage characteristic shifts toward curve I and if the ambienttemperature falls the characteristic shifts toward curve 5 the biasshifting with it along the line aob..

If a circuit such as that shown in Fig. 4 is used,

the shift in locus of the voltage-current characteristic of the negativeresistance thermistor with ambient temperature change can be held towithin closer limits such as curve 4 for low values and curve 2 for highvalues. In this circuit all elements similarly numbered perform the samefunctions as in Figs..2 and 3. A heater winding I 3 shunted by anambient temperature control thermistor T2 is connected across thebattery 7 in series with resistance I5. Heater winding I3 is-woundaround thermistor T1, being electrically but not thermallyinsulated from it to elevate its temperature to a level somewhat aboveambient temperature to efiect closer temperature regulation. Now, if theambient temperature changes, the resistance of T2 changes with itthereby changing the current through the heater winding which reducesthe tendency of the temperature of T1 to change, thus locus of itsvoltage-current characteristic. An in put transformer l2 couples theinput circuit and the signalsdiminishing the-shift in v although thebias operating point shifts along line aob, the sum of the resistancesof thermistors T1 and IE; remains nearly constant even though the slopeof the voltage-current characteristic changes appreciably.

. Fig. 6 illustrates a circuit' whereby the bias may be controlled insuch a way that the operating point will follow a locus that intersectsthe shifting voltage-current characteristics at points at which theslope is very nearly the same. ferring again to Fig. 1, if the bias ismade to vary as the ambient temperature varies in such a way that theoperating point shifts along aob' instead of (tab the negativeresistance of T1 and hence the gain will remain constant, Tris a slowacting thermistor operating as an ohmic resistance to adjust the bias.It has a negative coeflicient of resistance and decreases the bias Whenthe temperature increases or increases it when the temperaturedecreases. Resistance I4 is inserted to properly proportion therelationship between the bias and the resistance of the positivethermistor.

Any two or even all three of these methods of controlling the negativeresistance of T1 may be combined to realize the close regulation.obtainable by their concurrent action; For example the heater may beused to decrease the ambient temperature variation (say to a range'ofcurves 2 to 4 instead of I to 5) and then one or both of the othermethods may be used for more accurate compensation. Also in case thegain compensated device of Fig. 5 has excessive distortion at somepoint, the bias variation control of Fig. 6 can be used to. make theoperating point vary over that locus of points on the voltage-currentcharacteristics whereat the distortion is the same (such as a"ob ofFig. 1) and either of the other methods used to further minimize thegain variations.

- Fig. 7 shows a circuit which uses both heater and bias control tofacilitate the controlling effect of thermistor Also one thermistor maybe designed to perform the function of both heater control and bias tochange, the shift in locus of the volt- Re-v The c ange in the replacedwith a series resonant circuit such as condenser I! in series withinductance l6, tuned to the midband frequency, in order to passfrequencies in the amplifier band but keep the impedance facing thethermistor high at low frequencies. In this particular circuit theamplifier thermistor direct current is allowed to pass through the highimpedance winding of. both transformers.

To facilitate band-pass operation an electrical network such as thoseshown in Figs. 9a and 9b designed to take account of the effectivereactive component of the amplifier thermistor (due to its thermalinertia) may be put in series with it. A resistance shunted by acondenser in series with the thermistor gives a fair compensation for afiat gain over a band of frequencies. If an impedance consisting of aninductance and resistance in series and a capacitance in parallel isconnected in series with the thermistor a considerably bettercompensation is obtained.

Self-oscillation will not occur in a negative resistance thermistorcircuit used for amplification purposes if the sum of the, thermistorimpedance and the total impedance facing it has a positive resistancecomponent at all frequencies. sumcient positive coefficient resistanceshould, there- .fore,.be provided in the loop containing the thermistorfor all embodiments of this invention The fact that all the amplifiercircuits embodying the invention-have been described with reference totransformer coupled load and source applied to said first thermistor asthe ambient temperature increases.

4. In an amplifying circuit for amplifying signals, a negativetemperature cocfiicient thermiscircuits in no way limits its applicationto such circuits. Resistance, capacitance and network coupling may beused, the complexity of such coupling depending in each instance on theparticular circuit application.

These specific and preferred embodiments illustrate the utility of myinvention which is defined by the following claims.

What is claimed is:

1. A semiconductor operated in its negative resistance region,electrically biased for optimum amplification and a thermistor connectedin said biasing circuit to tend to cause said bias to be always optimumas the ambient temperature changes.

2. A thermistor having a negative resistance that varies with ambienttemperature and means I for operating it in the negative resistance partof its range'including electrical biasing means and means forcontrolling said negative resistance by changing the bias, said means sooperating that the change in negative resistance it introduces is equaland opposite to the change introduced by the change in ambienttemperature.

3. In an amplifying system, a source of waves to be amplified, a loadcircuit, and a negative temperature coeflicient thermistor included incircuit with said source and said load circuit, means to supply a biascurrent to said thermistor ofsuch value as to bias the thermistor intothe 7 negative resistance region of its volt-ampere characteristic, saidthermistor having a sumciently small temperature lag to enable itstemperature to vary at the frequency of said waves to be amplified. andmeans toreduce the effect tor that is temperature responsive at thefrequencies of said signals, a source of steady bias current for biasingsaid thermistor into the nega... tive resistance region of itscharacteristic, and means to compensate for variable amplification withvarying ambient temperature comprising a relatively slow-speedthermistor connected in circuit with said first thermistor and saidsource to change the bias current in response to change in ambienttemperature.

5. In combination, an amplifier comprising a thermistor having a speedof temperature response sufficiently high to follow variations to beamplified, means to bias said thermistor into the negative resistanceregion of its voltage-current characteristic, said characteristic havingdifierent slopes at different operating temperatures and the same valueof bias and means to compensate for the 'efiect on the, amplifyingproperties of the thermistor of changes in ambient temperaturecomprising means responsive to ambient temperature variations forchanging the amount of bias on said thermistor in the direction to tendto maintain at constant value the slope of the portion of thecharacterisic over which the thermistor is operated by said variationsto be amplified.

6. In an amplifier for electrical waves a nega-- tive temperaturecoeflicient thermistor capable of having its temperature changed at thefrequency of said waves, means 'to bias said thermlstorinto the negativeresistance region of its voltage-current characteristic, and meansresponsive to changes in ambient temperature for decreasing the value ofbias as the ambient temperature inperature response to heating currentto have its temperature and therefore its resistance changed at thefrequency of said waves, a sourceof bias current connected to pass acurrent through said thermistor of such magnitude as to enable the ofambient temperature variations upon the amplifying action of said systemcomprisin a secthermistor isexhibit a negative resistance effect to saidwaves, and a slow speed thermistor connected in circuit between saidsource of bias current and said first thermistor and responsive torelatively slow changes in ambient temperatures aifecting said firstthermistor and, in response to such changes inambient temperature,changing the amount of bias current and thereby maintaining the negativeresistance eifect exhibited by said first thermistor'substantlallyconstant.

'YLU'I'HER w. nossnv.

